This is not a joke. Read the last few lines, too.
So if you send a Christmas greeting, you might want to put something other than "Merry Christmas" in the subject line.
Jane

I just thought you all might like an immediate notice on this virus, it is particularly deadly.

VIRUS ALERT!! http://www.snopes.com/computer/virus/zafi.asp

You should be alert during the next days:
Do not open any message with an attached file called 'Merry Christmas'
regardless of who sent it, It is a virus that opens as an Open Log Fire
and will burn the whole hard disc in your computer.

This virus will be received from someone who has your e-mail address
in his/her contact list, that is why you should send this e-mail to all
your contacts. It is better to receive this message 25 times than to
receive the virus and open it.

If you receive a mail called 'Merry Christmas', though sent by a friend, do
not open it and shut down your computer immediately. This is the worst
virus announced, it has been classified by Microsoft as the most
destructive virus ever.

This virus was dis covered by McAfee yesterday, and there is no repair
yet for this kind of virus. This virus simply destroys the Zero Sector
of the Hard Disc, where the vital information is kept.